Various mobile electronic gears (cell phones, mobile information terminals PDA, note-type personal computers, DVD players, CD players, MD players, digital cameras, digital video cameras, etc.) comprise pluralities of DC/DC converters as power conversion apparatuses for converting the voltage of contained cells to operation voltage. In note-type personal computers, for instance, DC/DC converters are arranged near digital signal processors (DSP), micro processing units (MPU), etc.
As one example of DC/DC converters, FIG. 16 shows a step-down DC-DC converter comprising as discrete circuits an input capacitor Cin, an output capacitor Cout, an output inductor Lout, and a semiconductor-integrated circuit IC including a switching device and a control circuit on a printed circuit board. By operating the switching device based on a control signal from the control circuit, output voltage Vout expressed by Vout=Ton/(Ton+Toff)×Vin, wherein Ton is a time period in which the switching device is turned on, and Toff is a time period in which the switching device is turned off, is obtained from DC input voltage Vin. Even with the variation of the input voltage Vin, stable output voltage Vout can be obtained by adjusting a Ton/Toff ratio. An LC circuit comprising an output inductor Lout storing and discharging current energy, and an output capacitor Cout storing and discharging voltage energy acts as a filter circuit (smoothing circuit) for outputting DC voltage.
An output inductor Lout widely used at present, as shown in FIGS. 18 and 19, comprises a conductor wire 230 wound around a magnetic core 220. Used for the magnetic core 220 is high-resistance ferrite such as Ni—Zn ferrite, Ni—Cu—Zn ferrite, etc., so that a conductor wire can be wound directly around it.
The operation voltage of LSI (large scale integration) constituting DSP and MPU has been decreasing to 2.5 V, and further to 1.8 V, to expand the usable time period of cells. Because of such decrease of operation voltage, the voltage margin of LSI is reduced relative to the variation (ripple) of the output voltage of DC/DC converters, so that LSI is more influenced by noise. The switching frequencies of DC/DC converters have been increased from conventional 500 kHz to 1 MHz or more to suppress ripple, resulting in designing IC operable at 5-20 MHz.
Higher switching frequencies reduce inductance required for an output inductor Lout, enabling the size reduction of the inductor and a power supply circuit. However, higher switching frequencies contribute to the reduction of conversion efficiency due to loss generated in switching devices and inductors. Although power loss by inductors is caused predominantly by the DC resistance of conductor lines and output current at low frequencies, AC resistance (AC resistance of conductor lines and core loss of ferrite) is not negligible at high frequencies.
Accordingly, to operate DC/DC converters efficiently at high frequencies exceeding 5 MHz, particularly at about 10 MHz, it is important to reduce the core loss of ferrite constituting inductors. The core loss of ferrite is determined by hysteresis loss, eddy current loss and residual loss. It is known that these losses depend on the magnetic properties (coercivity, saturation magnetization, magnetic domain wall resonance, etc.), crystal grain size, resistivity, etc. of ferrite.
Inductors are also required to have stability under stress (little variation of inductance and less increase in loss under stress). Stress is caused by the difference in a linear thermal expansion coefficient between an inductor and a printed circuit board, the deformation of a printed circuit board, the curing of a molding resin when an inductor is sealed with a resin, shrinkage difference when internal conductors and ferrite are simultaneously sintered to produce a laminated inductor, the plating of external terminals, etc. Also, because DC/DC converters are exposed to heat generated by semiconductor-integrated circuits IC, etc., inductors used therein are required to exhibit stable characteristics at use temperatures; little variation of inductance with temperature.
As ferrite having improved stability under stress and temperature characteristics, JP 05-326243 A discloses Ni—Cu—Zn ferrite comprising 100% by mass of main components comprising 46.5-49.5% by mol of Fe2O3, 5.0-12.0% by mol of CuO, and 2.0-30.0% by mol of ZnO, the balance being NiO, and sub-components comprising 0.05-0.6% by mass of Co3O4, 0.5-2% by mass of Bi2O3, and 0.1-2% by mass in total of SiO2 and SnO2. However, this Ni—Cu—Zn ferrite contains as a sintering aid Bi2O3 having a melting point of 820° C. in a large amount of 0.5-2% by mass, though it contains SnO2 and Co3O4. Accordingly, it has an average crystal grain size of 3 μm or more, and large core loss and relative temperature coefficient αμir at high frequencies.
JP 2002-255637 A discloses a magnetic oxide ceramic composition comprising 100 parts by weight of main components comprising 45.0-49.5% by mol of Fe2O3, 1.0-30.0% by mol of ZnO, and 8.0-12.0% by mol of CuO, the balance being NiO, and 1.5-3.0 parts by weight (as SnO2) of Sn oxide, 0.02-0.20 parts by weight (as Co3O4) of Co oxide, and 0.45 parts or less by weight (as Bi2O3) of Bi oxide. However, it suffers as large a relative temperature coefficient of initial permeability as ±500 ppm/° C. between −25° C. and +85° C., and large core loss at high frequencies because of as much Sn oxide as 1.5 parts or more by weight as SnO2.